In the manufacture of lead acid storage batteries, it is customary to mold lead bushings in the plastic cover of the battery and to thereafter fuse the bushings to posts of the battery cell assemblies. It has long been the practice to melt the ends of the posts and bushings by means of manually operated acetylene torches. Not only does manual acetylene torch burning of the posts and bushings fail to lend itself to use in fully automated battery production lines, the quality and depth of the fused areas cannot be uniformly controlled.
While various proposals have been made for automatically fusing battery terminal posts and cover bushings by means of acetylene torch heating, electrical resistance heating, electrical induction heating, and tungsten inert gas welding, such proposals have faced various drawbacks, including the inability to obtain reliable fusion depths within the requisite processing time, undesirable melting of the cover about the bushings, and unacceptable appearance of the finished terminals.
Although fusion of battery terminal posts and bushings by means of induction heating has been found to be particularly advantageous, conductive losses and inefficiencies in the design of the induction heating coils and their supporting bus bar structures can increase the heating cycle time necessary for achieving adequate fusion. Longer heating cycles, in turn, increase the risk of damage to or weakening of the seals between the bushing and plastic cover, which may render the battery defective or create a potentially dangerous condition during usage.
Moreover, while it is desirable to simultaneous fuse a plurality of posts and bushings in automated production lines, conducive losses and difficulties in obtaining uniform fusion depths can be compounded by increased numbers of induction heating coils that are simultaneous operated. Heretofore, it has not been possible to achieve satisfactory fusion with more than two simultaneously operated induction heating coils, which again can impede automated production lines. Prior efforts to fuse four or more terminals simultaneously has resulted in underheating of some of the terminals and overheating and damage to other of the terminals. Increased conductive losses also may necessitate greater cooling of the coil supporting bus bar structures, thereby increasing the complexity of the apparatus.